It has been reported that tobacco withdrawal syndrome contributes to discouraging success rates after cessation of smoking. Studies have indicated that withdrawing from tobacco, and specifically from the drug nicotine, which is known to be the addictive, active agent that smokers seek, produces a variety of signs and symptoms described as tobacco withdrawal syndrome. The research question is : What is the effect of endogenous opioid blockade with naltrexone on 1) smoking behavior and 2) dysphoric states in human smokers?